Crystalline, amorphous, and elastic polypropylenes are known. Crystalline polypropylenes are generally regarded as comprising predominantly isotactic or syndiotactic structures and amorphous polypropylene is regarded as comprising predominantly of an atactic structure. U.S. Pat. Nos. 3,112,300 and 3,112,301 both of Natta, et. al. describes isotactic and prevailingly isotactic polypropylene.
U.S. Pat. No. 3,175,199 to Natta et al. describes an elastomeric polypropylene which can be fractioned out of a polymer mixture containing prevailingly isotactic and atactic polypropylenes When separated from the polymer mixture, a fraction of this polymer showed elastomeric properties which were attributed to a stereoblock structure comprising alternating blocks of isotactic and atactic stereosequences. U.S. Pat. No. 4,335,225 discloses a fractionable elastomeric polypropylene with a broad molecular weight distribution.
Elastomeric polypropylenes with narrow molecular weight distributions are also known which are produced in the presence of bridged metallocene catalysts. Polymers of this type were described by Chien et. al. in (J. Am. Chem. Soc. 1991, 113, 8569-8570), but their low melting point renders them unsuitable for certain applications. In addition, the activities of these catalyst systems are low.
U.S. Pat. No. 5,594,080 discloses an unbridged, fluxional metallocene catalyst system useful for the production of elastomeric polyolefins. These fluxional, unbridged catalysts can interconvert between geometric states on the time scale of the growth of a single polymer chain in order to produce isotactic, atactic stereoblock polyalphaolefins with useful elastomeric properties. Polyolefins produced with these fluxional catalysts systems can have a range of properties, from amorphous gum elastomers to useful thermoplastic elastomers to non-elastomeric thermoplastics.
The commercial utility of a catalyst system is closely tied to the polymerization activity. Processes that lead to an increase in activity of a polymerization system are of considerable practical utility. The activity of a polymerization system can in some cases be influenced by additives to the polymerization system. For example for both classical Ziegler-Natta systems as well as metallocene systems, the addition of hydrogen can result in an increase in propylene polymerization activity, see Pasquet, V., et al., Makromol. Chem. 1993, 194, 451-461 and references cited therein. One of the explanations for the hydrogen effect is the reactivation of the dormant sites resulting from 2,1-propylene misinsertions, see Corradini, P., et al., Makromol. Chem., Rapid Commun. 1992, 13, 15-20; Corradini, P., et al., Makromol. Chem., Rapid Commun. 1992, 13, 21-24; and Busico, V., et al., Makromol. Chem., Rapid Commun. 1993, 14, 97-103. Since hydrogen is also a chain transfer agent, the addition of hydrogen decreases the molecular weight, which limits the practical utility of the hydrogen effect where high molecular weight polymers are desired.
Activation of ethylene polymerization systems by the addition of small amounts of an alpha olefin is also known, see for example Brintzinger, H., et. al. Angew. Chemie, Int. Ed. Engl. 1995, 34, 1143-1170. This so-called "comonomer effect" (see Spitz, R., et al. Makromol. Chem. 1988, 189, 1043-1050) is useful in a process for the synthesis of ethylene polymers, but not for alpha olefin polymers. Hefert, N., et. al. Makromol. Chem. 1993 194, 3167-3182 report no effect of hexene on the rate of propene polymerization with a metallocene catalyst. Several explanations have been forwarded to explain this "comonomer effect" including a "trigger mechanism" (Ystenes, M., Makromol. Chem. "Macromolecular Symposia" 1993, 66, 71-81) and improved rates of diffusion due to the solubilization of active centers by incorporation of comonomer (see Koivumaki, J., et al. Macromolecules 1993, 26, 5535-5538).
Activation of propylene polymerization systems in the presence of 5% ethylene have been previously reported for magnesium chloride supported Ti-based catalysts by Spitz, R., et al. in Makromol. Chem. 1988, 189, 1043-1050 and in Spitz, R., et al. in "Transition Metal Catalyzed Polymerization", Quirk, R. P., Ed., Cambridge Univ. Press 1988, pp. 719-728, and with V-based Ziegler catalysts by Valvassori, A., et al. in Makromol. Chem. 1963, 61, 46-62. While such "synergistic effects" have been observed with classical Ziegler-Natta catalyst systems, Koivumaki et. al. point out that such synergistic effects do not work for homogeneous metallocene systems (see Koivumaki, J., et al. Macromolecules 1993, 26, 5535-5538).
Accordingly, there is a need for processes to improve the activity of metallocene catalysts systems capable of producing elastomeric polypropylenes of high molecular weight with high melting points.